1. Field of the Invention
This invention generally relates to a method and an apparatus for processing a digital audio signal. This invention particularly relates to a method and an apparatus for adding high-frequency-corresponding components to a digital audio signal to enhance the quality of audio contents represented by the digital audio signal.
2. Description of the Related Art
Typical conversion of an analog audio signal to a digital audio signal has a step of periodically sampling the analog audio signal at a prescribed sampling frequency to generate analog samples, and a step of changing the analog samples into corresponding digital samples constituting the digital audio signal. According to the Nyquist Theorem, the frequency of audio contents represented by the digital audio signal is limited to lower than the Nyquist frequency equal to a half of the sampling frequency.
The CD (compact disc) standards prescribe that a digital audio signal which results from analog-to-digital conversion of an analog audio signal at a sampling frequency of 44.1 kHz and a quantization bit number of 16 should be recorded on a CD. Therefore, the frequency of audio contents represented by the digital audio signal recorded on the CD is limited to lower than 22.05 kHz (the Nyquist frequency). Accordingly, components of the analog audio signal which have frequencies of 22.05 kHz or higher are lost during the analog-to-digital conversion to generate the digital audio signal. In general, such high-frequency components cause good presence or reality when audio contents represented by the analog audio signal are reproduced. Since the digital audio signal lacks high-frequency-corresponding components, audio contents reproduced from the digital audio signal tend to be poor in presence or reality.
U.S. Pat. No. 7,024,260 corresponding to Japanese Patent No. 3659489 discloses a method and an apparatus for shaping the waveform represented by a digital audio signal to compensate for lost high-frequency-corresponding components. In the method and the apparatus of U.S. Pat. No. 7,024,260, maximal and minimal values represented by samples of a digital audio signal are detected. A number of samples from a sample representing a minimal value to a maximum-value-corresponding sample is detected. A number of samples from a sample representing a maximal value to a minimal-value-corresponding sample is detected. Calculation is given of a first difference between the maximal-value-corresponding sample and the immediately-preceding sample. Calculation is given of a second difference between the minimal-value-corresponding sample and the immediately-preceding sample. First and second coefficients are calculated from the detected sample numbers. The first coefficient and the first difference are multiplied to generate a first multiplication result. The second coefficient and the second difference are multiplied to generate a second multiplication result. The maximal value represented by the maximal-value-corresponding sample is incremented by the first multiplication result. The minimal value represented by the minimal-value-corresponding sample is decremented by the second multiplication result.
According to the above signal processing in the method and the apparatus of U.S. Pat. No. 7,024,260, odd-order harmonics are added to the audio contents of a digital audio signal while even-order harmonics are not. Generally, concerning human hearing sense, odd-order harmonics with respect to a fundamental note tend to be uncomfortable and irritant to listeners. On the other hand, even-order harmonics tend to be comfortable. Therefore, in certain cases, audio contents reproduced from a digital audio signal generated by the method and the apparatus of U.S. Pat. No. 7,024,260 are uncomfortable to listeners.
Japanese patent application publication number 07-334180 corresponding to Japanese Patent No. 2888138 discloses an effect sound generating apparatus including a positive half-wave detecting circuit and a negative half-wave detecting circuit which receive an input audio signal. The positive half-wave detecting circuit and the negative half-wave detecting circuit divide the input audio signal into positive half waves and negative half waves. A first delay element defers the positive half waves by a first delay time to generate delayed positive half waves. A second delay element defers the negative half waves by a second delay time to generate delayed negative half waves. The first and second delay times can be determined independently of each other. A first attenuator damps the delayed positive half waves at a first attenuation to generate attenuated positive half waves. A second attenuator damps the delayed negative half waves at a second attenuation to generate attenuated negative half waves. A third attenuator damps the input audio signal at a third attenuation to generate an attenuated audio signal. The first, second, and third attenuations can be determined independently of one another. An adder sums the attenuated positive half waves, the attenuated negative half waves, and the attenuated audio signal to generate effect-sound-added audio signal. The effect-sound-added audio signal takes a synthesized waveform which can be selected from various waveforms in accordance with a combination of the first and second delay times, and the first, second, and third attenuations.
In the apparatus of Japanese application 07-334180, since the positive half waves and the negative half waves are processed separately, an amount of total signal processing tends to be great.